The invention is directed to optical telecommunications networks, and in particular, to a line amplification system for wavelength switched optical networks.
The equipment of an optical network can be generally classified into two categories, namely the switching nodes and the line system. The switching nodes are concerned with switching the channels in the input WDM (wavelength division multiplexing) signal to an output of choice, and with add/dropping the on-ramp/off-ramp user signals into/from the WDM signal. The line system includes the optical components and the fiber between two successive switching nodes, and is concerned with conditioning (line amplification, power control, dispersion control, etc.) the WDM signals to achieve long-haul transmission. Generally, the switching nodes may also include a preamplifier and a postamplifier, which are part of the line system.
Optical network architecture
Current optical networks are based on a WDM physical layer, using point-to-point (pt-pt) connectivity. While ultra-long reach achieved lately provides lower cost networks by substantially reducing the number of line regenerators, regeneration is nonetheless performed for all channels at the switching nodes, as often called xe2x80x98hidden regenerationxe2x80x99. This is because point-to-point connectivity implies OEO (optical-to-electrical-to-optical) processing of all channels arriving at a switching node. While optical-to-electrical O/E and E/O conversions are necessary for the off-ramp and on-ramp signals, they are not always necessary for the signals that pass through a switching node. The passthrough traffic, which is unnecessarily OEO processed, accounts for a large percentage (over 50%) of the total traffic at a node. As the number of channels in the WDM signal grows, the cost of the xe2x80x98hidden regeneratorsxe2x80x99 also grows, hindering the profit for the network provider.
The present invention is applicable to a wavelength switched network where each signal travels between a different source and destination node, without unnecessary OEO conversions at all intermediate nodes. The present specification is concerned with the line amplification system of such a network, that is generally described in the co-pending patent applications xe2x80x9cArchitecture for a Photonic transport Networkxe2x80x9d (Roorda et al.), Ser. No. 09/876,391, filed on Jun. 8, 2001. The present invention is also concerned with a line control system generally described in the patent application xe2x80x9cMethod for Engineering connections in a dynamically Reconfigurable Photonic Switched Networkxe2x80x9d (Zhou et al.), provisional patent application filed Jul. 18, 2001, Ser. No. 60/306,302, formal patent application filed August 2001, Ser. No. 09/930,528. This patent application claims priority from both above-mentioned patent applications. Details about the software architecture and operation of this photonic network are also described, illustrated and claimed in the co-pending provisional patent application xe2x80x9cArchitecture for an Optical Network Managerxe2x80x9d (Emery et al.), Ser. No. 60/298,008, filed on Jun. 13, 2001, which is incorporated herein by reference.
To summarize, the conventional architecture is replaced by a new architecture where repetitive regeneration of all channels in a WDM signal is not necessary, regeneration being performed only for individual channels based on the current network performance. Thus, the challenges in designing a line amplification system for such a network are substantially different from those encountered in conventional transport networks. For example, the number of the channels in a WDM signal on any link of such a network, as well as the bandwidth of the WDM signal, change as channels are arbitrarily added and removed across the network. As well, traditional channel performance optimization methods cannot be applied to end-to-end connections that pass through many nodes without OEO conversion.
Thus, there is a need to provide a line amplification system adaptable to condition a WDM signal with a variable number of channels, variable wavelength-to-channel allocation, and random channel add/drop.
There is also a need to provide a line amplification system that allows for use of OEO regeneration only at the nodes, and only for specific channels that need regeneration, based on the current network connectivity and performance.
There is also a need to provide a line amplification system with a line control system adapted to collect current information on current physical performance parameters of the span and link, to allow for individual channel optimization in the context of dynamic configuration and reconfiguration of the network.
Long reach and ultra-long reach optical transmission
Expansion of long haul optical communication networks has been fueled by the data traffic, and is estimated to be in the order of 70-150%. Particularly, since the popularity of the World Wide Web has enabled business transactions over the Internet, IP (Internet Protocol) and IP-based services have grown and evolved dramatically.
The reach, or the distance traveled by an optical channel along a path between a source node and a destination node, is limited by the combined effect of attenuation and distortion experienced by the signal along the path.
A solution to control attenuation is to place electro-optic repeaters (regenerators) at distances of 40-80 km, for retiming, regenerating and reformatting the optical signal. This solution however implies conversion of the optical signal to an electrical format and re-conversion of the processed electrical signal into an optical format (OEO conversion). With the advent of WDM, the cost of regenerators became prohibitive; this lead to development of optical amplifiers, which amplify an entire transmission band, i.e. a plurality of channels passing through it, without OEO conversion.
There are three types of optical amplifiers: post-amplifiers that connect to a transmitter to boost the output power, line amplifiers that amplify the optical signals along the signal route, and preamplifiers that improve the sensitivity of optical receivers. These different types of amplifiers provide different output power levels, use different input power levels, and generally have different noise figure requirements. The typical distance between two optical amplifiers is 80-100 km.
Although the EDFAs can support very long fiber spans by significantly increasing the optical power of all optical channels passing through them, they exhibit a wavelength-dependent gain profile, noise profile, and saturation characteristics. Hence, each optical channel experiences a different gain along a transmission path. The gain tilt is controlled typically, by selecting the channels of the WDM signal to have a similar gain tilt; however, this is not always possible, especially for networks with a high density of channels. Another solution used lately is to provide the optical amplifiers with dynamic gain flattening means such dynamic gain equalizers (DGE), which flatten-out specific wavelengths and can be tuned as needed.
For transmission speeds over 2.5 Gb/s, signal corruption caused by Chromatic Dispersion (CD) also becomes very important. Chromatic dispersion is the dependence of the speed of light on its frequency (wavelength), measured in ps/nm, and is attributable to optical fiber and optical components in general. CD compensation is realized by installing devices with a net CD in the opposite sense. For example, if a network provider wishes to compensate for 1700 ps/nm of CD for a particular wavelength or a set of wavelengths, it can use a dispersion compensating module (DCM) that has a negative value of xe2x88x921700 ps/nm of CD in the same wavelength regime. After the compensator, the CD is essentially zero. Sometimes the network provider will compensate the net dispersion to a non-zero value.
Another way to increase the signal reach is to use the Stimulated Raman Scattering effect. Thus, by pumping the fiber using a laser of a certain power(s) and wavelength(s), the signal is further amplified by this effect. Typically, the Raman pump injects light in a direction opposite to the traffic flow; pumping in the forward direction is also possible. The spectral intensity profile of the Raman gain is dependent on the power and wavelength of the reverse-pumped light and also on the number of the wavelengths (pumps) used. The broader the spectrum of the pumped light, the wider the spectral intensity profile of the gain (i.e. the number of traffic channels amplified) is. However, the complexity of control increases with the number of the pumped wavelength; also these wavelengths need to be selected so as to not interfere with the traffic and the supervisory (service) channels. As Raman scattering phenomena produces gain at wavelengths higher than the pump wavelength, the wavelengths of the Raman pumps depend on the transmission band used for traffic.
As a result of the above methods of increasing the transmission reach, distances of over 3,000 km were obtained lately experimentally, and research for increasing this distance continues.
Nonetheless, in traditional networks, channel allocation is fixed and therefore any reach-capacity enhancement needs to be performed at regular intervals and on a span-by-span basis. This results in a very large service activation time. Furthermore, performance of the line amplification system is enhanced using span equalization, meaning that the power of channels co-propagating along the same fiber span is adjusted based on the power of the worst performing channel. This is clearly not an efficient way of utilizing the network resources.
There is a need for a line amplification system that allows channels originating at arbitrary nodes in the network to travel over a long distance to an arbitrary destination node. Such a line amplification system will need to condition the channels based on current physical performance parameters along a span and a link, to allow for individual channel optimization in the context of dynamic configuration and reconfiguration of the network.
A typical optical network is characterized by different losses in each section, depending upon the fiber type, fiber length, cabling and slicing losses. Also, different network operators have distinctive losses and loss distribution in their networks. Currently, enhancement of each span performance is addressed differently, resulting in a plurality of hardware variants, with the ensuing complexity in inventory management and additional costs.
There is a need to provide a line amplification system that is modular, scalable and flexible in performance, for minimizing the number of hardware variants, the costs associated with the complexity of inventory management and installation and operation costs.
The current networks are able to maintain inventory data at the network element level, using complex software running on a network management system, if available. They are not able to report the specific configuration at the unit, card-pack and shelf, bay and network element level. For large networks, there is currently a huge challenge to maintain an updated view of the network inventory; this results in lengthy processes for upgrades, maintenance and repairs.
There is a need to provide a line amplification system adapted to maintain current network topology and connectivity information to allow for real-time span and link optimization as the network grows.
It is an object of the invention to provide a line amplification system for an ultra-long haul photonic network capable of automatic optical routing and switching of traffic. It is another object of the invention is to provide a line amplification system that is modular, scalable and flexible in performance.
Still another object of the invention to provide a flexible line control system where each wavelength is engineered individually for allowing application-specific capacity-reach trade-off, with no changes to the hardware configuration of the line amplification system.
The invention provides an optical amplifier for a wavelength switched optical network comprising a Raman unit for amplifying a WDM optical signal with a Raman gain; an EDFA unit connected to the Raman unit for further amplifying the WDM signal with a EDFA gain; and a shelf-level control network for monitoring and controlling operation of the optical amplifier to maintain a substantially similar power for all channels of the WDM signal.
According to another aspect of the invention, a line amplification system for a wavelength switched optical network comprises at a first flexibility site, a post-amplifier unit for amplifying a WDM optical signal and launching same over a fiber link; at a second flexibility site, a pre-amplifier unit for amplifying the WDM optical signal received over the fiber link; one or more line amplifier units connected on the fiber link between the first and second flexibility sites for amplifying the WDM signal; and a line monitoring and control system for collecting a plurality of real-time operational parameters pertinent to the current operation of the units and operating the line amplification system according to a plurality of target operational parameters, wherein the real-time operational parameters change due to end-to-end network churn caused by dynamic set-up and tear-down of user connections.
Still further, the invention relates to a line monitoring and control system for a line amplification system of a wavelength switched optical network comprising: an embedded control layer, comprising an embedded controller provided on each card pack of an optical amplifier for controlling operation of the card pack; a link control layer comprising a plurality of shelf processors for coordinating operation of all optical amplifiers connected on a link of the wavelength switched optical network to achieve an output power profile target for the link; and a network control layer comprising a plurality of optical link controllers for coordinating operation of all optical modules placed on a plurality of consecutive links making-up a connection.
A control loop for an optical amplification span of a wavelength switched optical network is also provided according to the invention. The control loop comprises: means for measuring at preset intervals, a set of performance data regarding a WDM signal traveling along an optical section; a vector gain loop for receiving a set of current performance data and a gain target, and providing a gain adjustment signal comprising a gain adjustment component for each channel of the WDM signal; a control rules block for processing the gain adjustment components according to the set of current performance data, a set of previous performance data and section status data, and providing a control signal; wherein the control signal adjusts the operational parameters of all card-packs of the optical section to provide substantially similar gain for each channel of the WDM signal.
According to a yet further aspect, the invention provides a method of transmitting a WDM signal along a span of a wavelength switched optical network comprising: measuring an input power of the WDM signal at the input of the span; amplifying the WDM optical signal and measuring the spectrum and output power of the WDM signal; and controlling operation of the optical amplifier according to the input and output power and spectrum and also according to a set of rules to compensate for the losses and degradation of the WDM signal along the fiber of the span.